Ferromagnetic thin metal film type magnetic recording mediums comprising a support having thereon a magnetic layer of a ferromagnetic thin metal film formed by vacuum film-forming methods such as a vacuum evaporation method, sputtering process, ion plating process, etc., have excellent electromagnetic characteristics in comparison with coated type magnetic recording mediums having a magnetic layer composed mainly of a ferromagnetic powder and a binder resin because the magnetic layer of the ferromagnetic thin metal film does not contain any binder resin. Accordingly, the ferromagnetic thin metal film type recording mediums are drawing attention as magnetic recording mediums for high-density recording and may be put to practical use.
Particularly, the vacuum evaporation method has advantages in that the method is a dry treatment system, the film forming rate is high, and the manufacturing process is relatively simple in comparison with other film forming methods under vacuum. Various proposals have been made for a long time to improve the ferromagnetic thin metal film type magnetic recording mediums prepared by the vacuum evaporation method. For example, such proposals are described in U.S. Pat. No. 3,342,632 and 3,342,633.
The ferromagnetic thin metal film type recording mediums have serious practical problems in regard to running properties, durability and corrosion resistance.
A magnetic recording medium and a magnetic head slide at a high speed when they are brought into contact with each other during the course of recording, reproduction and erasing of signals during magnetic recording. During these operations, it is required that the magnetic layer of the magnetic recording medium is not abraded and broken. That is, the magnetic layer is run smoothly and stably.
JP-A-60-69824 (the term "JP-A", as used herein, means an "unexamined published Japanese patent application") and JP-A-60-85427 disclose a method wherein a layer containing a thermoplastic resin, a thermosetting resin, a fatty acid, a metal salt of a fatty acid, a fatty acid ester or an alkylphosphoric ester as a lubricating layer or a protective layer for the magnetic layer of the ferromagnetic thin metal film type recording medium is formed on the surface of the magnetic layer to meet the above requirements.
Many methods using fluorine-containing compounds as a material for the protective layer have been proposed. For example, JP-A-61-107528 discloses that compounds having a branched perfluoroalkenyl group are used to improve durability. U.S. Pat. No. 3,778,308 discloses a method using perfluoroalkyl polyether compounds.
JP-B-60-10368 (the term "JP-B", as used herein, means an "examined Japanese patent publication") discloses a method wherein perfluoroalkyl polyether compounds having modified terminal groups obtained by modifying one or both terminals of a perfluoroalkyl polyether molecular chain with a polar group such as a carboxyl group are used as compounds for the protective layer.
The use of multi-chain perfluoroalkyl polyether amides obtained by the dehydration-condensation of a compound having two or more amino groups with a carboxylic acid having a perfluoroalkyl polyether chain as a lubricant for disc type ferromagnetic thin metal film type magnetic recording mediums has been disclosed (Sugiyama, et al., Proceedings (page 425) of the 34th All Japan Meeting of the Lubricant Society of Japan, B.28).
Further, JP-A-64-33713, JP-A-1-112516, JP-A-3-102614, JP-A-1-112528 and JP-A-62-192029 disclose that compounds having a polar group such as an isocyanato group or an oxo acid group can improve durability.
Among the above-described compounds for the protective layer, the perfluoroalkyl polyethers having modified terminals have a particularly excellent lubricating effect, and are useful as the material for the protective layer of the ferromagnetic thin metal layer type magnetic recording medium.
The magnetic recording mediums have been used in many places under varying environmental conditions in recent years. Thus, it has been demanded that the magnetic recording mediums have good running durability over a wide range of environmental conditions (i.e., wide varieties of temperature and humidity conditions) ranging from high-temperature and high-humidity conditions to low-temperature and low-humidity conditions. Conventional techniques cannot always sufficiently cope with these requirements.
Further, when the problem of corrosion resistance is taken into consideration, conventional techniques are considered to be more insufficient under the above working environmental conditions.
Since the magnetic layer is a thin metal film, corrosion resistance is an essential problem of the ferromagnetic thin metal film type magnetic medium in that the magnetic layer itself is deteriorated by the effects of oxygen and water in the air during storage.
The improvement in corrosion resistance is directed to minimizing and/or solving the following problems;
(1) Formation of rust from water condensed and dried on the ferromagnetic thin metal film;
(2) Gradual decrease in the amount of magnetic flux during storage under high temperature and humidity conditions (demagnetization);
(3) Formation of rust on the surface of the ferromagnetic thin metal film by the effects of sulfur dioxide and nitrogen oxide gases present in air; and
(4) Deposition of salts such as NaCl, MgCl.sub.2, etc., contained in seawater in coastal regions on the surface of the ferromagnetic thin metal film and subsequent formation and spreading of rust from the deposited or starting point (salt damage).
With regard to the above items (1), (2), and (3), various methods have been proposed for improving the corrosion resistance of the ferromagnetic thin metal film to a level which causes no practical trouble. With regard to the salt damage (item 4), however, an appropriate method has not been found as yet.
Further, it was found that the methods for solving the problems of the above items (1), (2), and (3) are not considered to be fully sufficient overall when all of the characteristics of the magnetic recording medium are taken into consideration. For example, some methods are effective in improving the corrosion resistance of the vapor deposition tape, but are not effective in other aspects or causes new problems.
For example, in a method wherein a protective layer comprising an inorganic material such as SiO.sub.2, C or TiO.sub.2 is provided on the ferromagnetic thin metal film, the protective layer must have a thickness of at least 300 .ANG. to obtain the desired corrosion resistance. Such a large thickness causes a lowering in reproduction output due to spacing loss and the lowering of output is not preferred in short wave recording regions where the thin metal film medium is used, in particular.
Further, in methods for imparting corrosion resistance to the ferromagnetic thin metal film by various after-treatments (for example, a method wherein the magnetic recording medium is allowed to stand in an atmosphere under given temperature and humidity conditions, a method wherein a passive film is electrochemically provided and a method wherein a vapor deposition web film is subjected to an oxidation treatment), corrosion resistance to water can be improved (the problems of the items (1) and (2)), but it is still insufficient to prevent rust from being formed on the ferromagnetic thin metal film by salts (salt damage).
Further, JP-A-58-26319, JP-A-58-23622, JP-A-58-60432, JP-A-59-63031 and JP-A-59-60738 disclose a method wherein the ferromagnetic thin metal film is brought into contact with ozone during the formation of the film or the ferromagnetic thin metal film is exposed to an ozone atmosphere after the formation of the film to thereby improve corrosion resistance. This method is remarkable in improving the corrosion resistance of the ferromagnetic thin metal film relative to other conventional methods.
However, these methods alone are still considered to be insufficient with respect to corrosion resistance against salt damage (the problem of the above item (4)).
Most methods for improving corrosion resistance, in particular, do not relate to an improvement in running properties.
JP-A-3-207019 discloses a method wherein a specific amount of a perfluoropolyether having a polar group is fixed to the surface of the ferromagnetic thin metal film to thereby improve running properties and durability. However, when the improvement in corrosion resistance against salt damage is more desirable or the working atmosphere with respect to high/low-temperature and humidity conditions becomes more severe, this method cannot provide the improvement in running durability because the manufacturing method and the specific amount thereof are apparently insufficient due to the formation of a dense protective film.
As described above, a method capable of providing a ferromagnetic thin metal film type recording medium excellent in electromagnetic characteristics which is best suited for high density recording has not been proposed yet. In recent years, conventional methods do not provide the desired results when the ferromagnetic thin metal film type recording medium is exposed to a wide range of working atmosphere conditions.
The present invention has been accomplished with a view to solving the above-mentioned problems.